Certain medical conditions can activate the narcissistic defense mechanism. Chronic ailments are likely to lead to the emergence of narcissistic traits or a narcissistic personality style. Traumas (such as brain injuries) have been known to induce states of mind akin to full-blown personality disorders. Such "narcissism", though, is reversible and tends to be ameliorated or disappear altogether when the underlying medical problem does. Other disorders, like the Bipolar Disorder (mania-depression) are characterised by mood swings that are not brought about by external events (endogenous, not exogenous). But the narcissist's mood swings are strictly the results of external events (as he perceives and interprets them, of course).
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)